1. Field of the Invention
The invention relates to a method for estimating the impulse response of a propagation channel, especially its parameters such as the delay values, the directions of arrival as well as the values of the complex amplitudes associated with these parameters, with an a priori knowledge of the signal.
It is applicable, for example, to the estimation of the parameters of a finite impulse response filter which can be written, though not necessarily so, in specular form, namely a filter that can be written in the form of weighted Diracs.
The invention can be used to estimate propagation channels in the field of radiocommunications but also, generally, it can be applied to any signal filtered by a finite impulse response linear filter.
In a transmission system, especially one using radio waves, a transmitter sends out a signal in a transmission channel to a receiver. The signal that is sent undergoes amplitude and phase fluctuations in the transmission channel. The signal received by the receiver consists of copies of the transmitted signal that are temporally shifted and modified. The fluctuations of the signal and the shifts give rise to a phenomenon known to those skilled in the art as inter-symbol interference. The interference arises especially out of the law of modulation used for transmission and also from the multi-path propagation in the channel.
The received signal generally arises out of a large number of reflections in the channel. The different paths taken by the sent signal thus lead to different delays in the receiver. The impulse response of the channel represents the totality of the fluctuations to which the sent signal is subjected.
The estimation of the propagation channel in a radiocommunications system is useful in several respects, some of which are indicated here below as examples.
The demodulators generally require knowledge of the channels in order to remedy the harmful effects that they have caused,
a second point of interest is that of urban or extra-urban localization, for example the principle of emergency localization using the number xe2x80x9c911xe2x80x9d in the United States
Finally, the knowledge of the propagation channels can also serve for the use of smart antennas at reception as well as transmission,
2. Description of the Prior Art
There are various techniques known in the prior art for estimating propagation channels and their parameters.
For example the document by R. Rick and L. Milsteil, xe2x80x9cPerformance acquisition in mobile ds-cdma systemsxe2x80x9d, IEEE Trans on Communications, Vol: 45 (No: 11):pp: 1466-1476, November 1997, proposes a search for propagation delays by using a bank of non-coherent detectors. The results are proposed for multi-path channels in the presence of Doppler phenomena and inter-cell and intra-cell interference.
The document by R. Rick et L. Milsteil, xe2x80x9cOptimal decision strategies for acquisition of spread-spectrum signals in frequency selective fading channelsxe2x80x9d in IEEE Trans. on Communications, Vol: 46 (No: 5):pp: 686-694, May 1998, discloses an optimal decision rule based on the outputs of the correlators proposed in the document referred to here above. A single-user technique of this kind is limited by interference in the case of multiple-users.
There also exist known ways of using rectangular shaping filters, for example by the method described in one of the following documents:
E. Strom, S. Parkvall, S. Miller, and B. Ottersen, xe2x80x9cPropagation delay estimation in asynchronous direct-sequence code-division multiple access systemsxe2x80x9d, IEEE Trans on Communications, Vol: 44:pp: 8-93, January 1996
S. Parkvall, xe2x80x9cNear-Far Resistant DS-CDMA Systems: Parameter estimation and Data Detectionxe2x80x9d, PhD thesis, Royal Institute of Technology Stockholm, Sweden, 1996.
S. E. Bensley and B. Aazhang, xe2x80x9cMaximum likelihood estimation of a single user""s delay for code division multiple access communication systemsxe2x80x9d, Conf. Information Sciences and Systems, 1994.
In the case of shaping filters with a duration greater than a chip time, these different methods are no longer suitable.
Algorithms for the combined estimation of angles of arrival and of the differential delay times, on known and received signals, based on sub-space techniques have been proposed, for example in the document by P. Gounon, xe2x80x9cAnalyse spatio-temporelle haute rxc3xa9solution à l""aide d""une antenne activexe2x80x9d, (High Resolution Space-time Analysis Using an Active Antenna) Traitement du Signal (Signal Processing), Vol. 11 (No. 5), pp. 351-360, 1994.
The document by A. J Van der Veen, M. C. Vanderveen, et A. J. Paulraj, xe2x80x9cJoint angle and delay estimation using shift-invariance propertiesxe2x80x9d, IEEE Sig. Proc Letters, Vol.4 (No.5): pp. 142-145, 1997, discloses methods for the estimation of the physical parameters of propagation by means of methods based on sub-spaces.
However, such methods suffer from a deterioration of performance characteristics once the impulse responses of the propagation channels are correlated. This situation occurs especially when the complex amplitudes do not vary with sufficient speed on the covariance matrix of the impulse responses estimated in terms of the least-error squares by means of the signal transmitted,
FIG. 1 shows the different techniques of maximum likelihood.
A maximum likelihood method has been proposed, for example, in one of the following references:
J. Grouffaud, xe2x80x9cIdentification spatio-temporelle de canaux de propagation à trajets multiplesxe2x80x9d, (Space-time Identification of Multi-Path Propagation Channels), PhD thesis, École Normale Supxc3xa9rieure de Cachan, June 1997.
M. Wax and A. Leshem, xe2x80x9cJoint estimation of delays and directions of arrival of multiple reflections of a known signal.xe2x80x9d, IEEE Trans. on Signal Processing, Vol: 45(No: 10):pp: 2477-248, October 1997.
but it does not deal with the MIMO (Multiple Input Multiple Output) context.
The document by P. Graffoulixc3xa8re, xe2x80x9cMxc3xa9thodes actives spatio-temporelles large bandexe2x80x9d (Active Wideband Space-Time Methods), published in  less than  less than Techniques et performances. Applications En Sonar greater than  greater than  (Techniques and Performance. Sonar Applications), PhD thesis, INPG, March 1997, also discloses a method of estimation based on maximum likelihood but the studies on performance deal only with the case of a single source or of several distinctly separate sources. A similar study is disclosed in the document N. Bertaux, xe2x80x9cContribution à l""utilisation des mxc3xa9thodes du Maximum de Vraisemblance en traitement radar actifxe2x80x9d (Contribution to the Use of Maximum Likelihood Methods in Active Radar Processing), PhD thesis, Ecole Normale Supxc3xa9rieure de Cachan, January 2000, for active radar applications in the case of single sources.
The present invention relates to a method that can be used especially to estimate the parameters of the propagation channel by working on the correlated signals, in selecting a certain number of samples and in searching for the values of the delay parameters and/or directions of arrival, for example, which would enable the most efficient reconstruction of the signal received.
The invention also relates to a method that integrates pulse compression techniques such as pre-processing.
The invention relates to a method for the estimation of one or more parameters of a propagation channel with a priori knowledge of the signal in a system comprising one or more sensors.
The method of the invention comprises the following steps:
correlating one or more signals x(t) received by the sensors with the known signal c(t),
sampling the signals at a sampling period Te and selecting a number of samples per concatenation,
determining at least one parameter of the propagation channel, including xcfx84 or xcex8, which enables the most efficient reconstruction of the signals x(t) via a maximum likelihood method.
According to one embodiment, the characteristics of the system of sensors are known and comprise for example:
a correlation step with a known signal c(t) equal to 1,
the signals received on the antenna being expressed in the form X=S(xcex8,xcfx84)h+B, and
the estimates of the parameters as xcfx84 and xcex8 being expressed in the following form:                               θ          ^                ,                              τ            ^                    =                    ⁢                      arg            ⁢                          xe2x80x83                        ⁢                                          min                                  θ                  ,                  τ                                            ⁢                                                "LeftDoubleBracketingBar"                                                            ∏                      S                      ⊥                                        ⁢                                          xe2x80x83                                        ⁢                                                                  (                                                  θ                          ,                          τ                                                )                                            ⁢                      X                                                        "RightDoubleBracketingBar"                                2                                                                            =                ⁢                  arg          ⁢                      xe2x80x83                    ⁢                                    min                              θ                ,                τ                                      ⁢                          {                                                X                  xe2x80xa0                                ⁢                                                      ∏                    S                    ⊥                                    ⁢                                      xe2x80x83                                    ⁢                                                            (                                              θ                        ,                        τ                                            )                                        ⁢                    X                                                              }                                          
where Π195 S is the projector orthogonal to the image generated by the column vectors of S(xcfx84, xcex8).
The characteristics of the system of sensors are for example the response to the antenna.
It may comprise a step for determining the complex amplitudes h of the impulse response of the propagation channel from the estimates of the estimated parameters as xcfx84 and xcex8.
According to another embodiment, the characteristics of the system of sensors are not known, and the method comprises for example:
a step for the correlation of the signals received by the network of sensors with a known signal c(t) equal to 1,
a step in which the signals received are expressed in a concatenated form Y="psgr"(xcfx84)xcex1+N where "psgr"(xcfx84) is equal to the convoluted product of the unit matrix IN and the matrix             S      ⁢              (        τ        )              =          ⌊                                    s            xe2x80x2                    ⁢                      (                          τ              l              1                        )                          ,                  …          ⁢                      xe2x80x83                    ⁢                                    s              xe2x80x2                        ⁢                          (                              τ                Pi                1                            )                                      ,        …        ⁢                  xe2x80x83                ,                  s          ⁢                      (                          τ              pU              U                        )                              ⌋        ,
and xcex1 contains the responses of the paths of the different users,
a step for the estimation of the delay vector xcfx84 from                               τ          ^                =                ⁢                  arg          ⁢                      xe2x80x83                    ⁢                                    min              τ                        ⁢                                          "LeftDoubleBracketingBar"                                                      ∏                    Ψ                    ⊥                                    ⁢                                      xe2x80x83                                    ⁢                                                            (                      τ                      )                                        ⁢                    Y                                                  "RightDoubleBracketingBar"                            2                                                              =                ⁢                  arg          ⁢                      xe2x80x83                    ⁢                                    min              τ                        ⁢                          tr              ⁡                              (                                                      Y                    xe2x80xa0                                    ⁢                                                            ∏                      Ψ                      ⊥                                        ⁢                                          xe2x80x83                                        ⁢                                                                  (                        τ                        )                                            ⁢                      Y                                                                      )                                                        
where Πxcexa8xe2x8axa5 is the projector orthogonal to the image generated by the line vectors of "psgr"(xcfx84).
The step of correlation of the signals is performed with a signal c(t) different from 1. The characteristics of the system of sensors are known and this correlation step comprises a step for the estimation of the parameters xcfx84 and xcex8 from:       θ    ,          τ      =              arg        ⁢                  xe2x80x83                ⁢                              min                          θ              ,              τ                                ⁢                                    X              ω                        ⁢                          R              b                              -                1                                      ⁢                                          ∏                Φ                ⊥                            ⁢                                                (                                      θ                    ,                    τ                                    )                                ⁢                                  X                  ω                                                                          where            ∏      Φ      ⊥        ⁢          =              I        -                              Φ            ⁡                          (                              θ                ,                τ                            )                                ⁢                                    (                                                                    Φ                    xe2x80xa0                                    ⁡                                      (                                          θ                      ,                      τ                                        )                                                  ⁢                                  R                  b                                      -                    1                                                  ⁢                                  Φ                  ⁡                                      (                                          θ                      ,                      τ                                        )                                                              )                                      -              1                                ⁢                                    Φ              xe2x80xa0                        ⁡                          (                              θ                ,                τ                            )                                ⁢                      R            b                          -              1                                          
The invention also relates to a device for estimating one or more parameters of a propagation channel with a priori knowledge of the signal in a system comprising one or more sensors.
The device comprises at least:
a device adapted to the correlation of the signal received by the sensor or sensors s(t) with a known signal c(t),
a device adapted to the selection of a number of samples of the signal obtained after the correlation step, and
a device adapted to the determining of the parameters of the channel by a maximum likelihood method.
The method can be applied, for example, in applications of the MIMO (Multiple Input Multiple output) or SIMO (Single Input Single Output) type.
The invention in particular has the following advantages:
in the cooperative MIMO context, the estimator, within the framework of a known antenna, makes use of a parametric model of the signals received on the antenna, unlike the deterministic maximum likelihood method which estimates each sample of the signal without any a priori assumptions,
in limiting the field of analysis in certain cases of application, it leads to a reduction of the size of the data processed, and thus results in faster processing.